1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, and more particularly to an evaporative fuel-processing system which has a function of detecting presence/absence of a leak from an evaporative emission control system of the engine.
2. Prior Art
Conventionally, evaporative fuel-processing systems for internal combustion engines have been widely used, which are so constructed that evaporative fuel generated in a fuel tank is temporarily stored in a canister and the thus stored evaporative fuel is suitably purged into an intake system of the engine.
To detect an abnormality occurring in an evaporative emission control system which is comprised of the canister, a passage connecting the canister and the fuel tank of the engine, a passage connecting between the canister and the intake system of the engine, the present assignee has already proposed a method which comprises negatively pressurizing the evaporative emission control system by means of negative pressure from the intake system of the engine, disconnecting the evaporative emission control system from the intake system of the engine when the evaporative emission control system has been properly negatively pressurized, and detecting presence/absence of a leak, based on a change in the pressure within the evaporative emission control system, for example, by Japanese Provisional Patent Publication (Kokai) No. 5-180093 to which U.S. Ser. No. 07/942,875 corresponds.
In the above proposed method, a pressure sensor is mounted in the fuel tank to detect pressure therein (tank internal pressure PT), and the evaporative emission control system is disconnected from the intake system of the engine at a time point the tank internal pressure PT is reduced to a predetermined negative pressure P1 (at a time point t4 at (d) in FIG. 2), followed by detecting the presence/absence of a leak, based on a rate of increase in the tank internal pressure, which is detected over a predetermined time period elapsed after the disconnection. Therefore, the proposed method has the following problem:
When the tank internal pressure PT decreases to the predetermined negative pressure P1, the pressure within the canister has already dropped to a value lower than the pressure P1 (see (d) in FIG. 2), so that the tank internal pressure PT continues to decrease even after the time point t4, and starts to increase only after a time point t5. As a result, if the rate of increase in the tank internal pressure is calculated based on the tank internal pressure PT detected at the time point t4, the resulting increase rate value is smaller than a normally required increase rate value (rate of increase between the time points t5 and t6), or the tank internal pressure at the time point t6 is lower than the predetermined negative pressure P1, which makes it impossible to detect a small degree of leak.